The strap of this type of seal generally has serrations or teeth formed along its length so that when it is passed through an aperture in the head part it cannot be pulled out. This seal, often known as a ‘pull-up seal’, provides a tie which can be put around the neck of a bag, and then pulled tight. Because the seal cannot be removed without cutting the tail or separating the head from the tail, the seal provides a tamper evident closure for bags which may contain valuable or sensitive material. Typically, the head part is printed with a unique number, so that one seal cannot be removed and replaced by a fresh seal without this replacement being evident.
Seals with straps which are not serrated are also known. The strap can thus be smooth and the head includes angled teeth or barbs which engage the strap if it is pulled back out of the aperture in the wrong direction.
Pull-up seals are typically used to seal cash bags, property bags and clinical waste bags, but can also be used to lock items other than bags, for example roll cages or tanker valves, to take just two examples.
In a simple version of this type of seal, the seal is moulded as one integral part, the head part having an aperture of specific dimensions which will allow the serrations on the tail to pass through in one direction, but prevent the tail from being pulled out. The serrations are usually formed with an inclined leading edge, so that they can pass under a catch within the head.
In another version of a pull-up seal, the aperture is formed through a separate metal or acetal resin insert which is moulded into a chamber in the head part during the injection moulding of the strap, in other words the chamber is injection moulded around the insert. This provides a seal with a high degree of security, as the locking part within the chamber is not susceptible to tampering. However, this process for manufacturing seals is relatively expensive and difficult to perform reliably.